This invention relates to the preparation of large particle size monodisperse latexes by seeded emulsion polymerization.
Considerable effort has been expended in recent years to develop large particle size monodisperse latexes as standards for instrument calibration and for other uses. By "monodisperse" particles is meant latex particles having coefficients of variation (standard deviation divided by number average diameter) of less than 2 percent. By large particle size is meant a particle size (number average diameter) of at least 2-30 microns or more, e.g., up to about 100 microns or more. Prior to the microgravity emulsion polymerization described in U.S. Pat. No. 4,247,434 to Vanderhoff et al, four approaches have been taken in attempts to prepare such latexes.
The pioneer approach was a successive seeded emulsion polymerization method developed by Vanderhoff et al., J. Opt. Soc. Am., 1954, 44, 603. In this process, a small monodisperse latex was used as the seed. To this latex was added monomer, a peroxy initiator, and an anionic emulsifier. The reaction mixture was then heated to the polymerization temperature, and by carefully controlling emulsifier concentration, nucleation of new particles was avoided. Each monodisperse latex thus produced was used as the seed for the next polymerization. It was found that the tolerable range of emulsifier concentration was broad at small particles sizes but became narrower with increasing particle size. The method produced monodisperse particles of particle size 0.1-2.0 microns with coefficients of variation of 5% at 0.1 micron, 1% at 0.2 micron, 0.5% at 1 micron and 0.75% at 2 microns. However, the amount of coagulum increased with increasing particle size above 2 microns, so that only 100 gram quantities of particle sizes of 5.6 microns were produced.
In the second technique the emulsion polymerization was carried out without added emulsifier, as reported by Matsumoto et al, Kobunshi-Kagaku, 1965, 22, 481. Apparently, the persulfate ion initiator introduced sulfate end groups into the polymer molecules which then acted as emulsifier to stabilize the particles. Particle size of the latexes was determined by the concentrations of monomer, initiator and electrolyte. This method produced monodisperse latexes of 0.1-1 micron particle size and has been claimed to produce particle sizes of up to 4 microns. Y. Chung-Li et al, Prog. Colloid Polym. Sci., 1976, 60, 163.
In the third technique, a microsuspension polymerization was used, similar to conventional suspension polymerization except for a higher concentration of stabilizer which gave a relatively broad distribution of smaller particle sizes, e.g., average size 1-100 microns. Illustrative of this method is the "limited coalescence" method of R. M. Wiley (U.S. Pat. No. 2,932,629) which uses as the suspension stabilizer lyophobic colloidal particles modified with a "promoter" to adjust the hydrophilic-hydrophobic balance so that they adsorb at the oil-water interface. This method has been used to prepare styrene-divinylbenzene copolymer particles of 5-100 micron average diameter with coefficients of variation of about 20%. However, these particles are not monodisperse and even if the samples are fractionated to give narrower-size fractions, the particle size distributions are still much broader than those of latexes prepared by seeded emulsion polymerization.
The fourth technique is a seeded emulsion polymerization using a two-step high-swelling ratio approach as described by J. Ugelstad et al, Adv. Colloid Interface Sci., 1980, 13, 101. The combination of a water insoluble compound and a water-miscible organic solvent as swelling agents gave high monomerpolymer swelling ratios, thereby allowing the preparation of large particle size latexes with fewer seeding steps. Latexes prepared by this method are said to have particle sizes up to 50 microns. However, the latexes comprise a main distribution of monodisperse particles with contaminating smaller and larger off-size particles, and it has been reported that these particles explode and rupture upon exposure to the electron beam in an electron microscope, possibly because of sudden vaporization of water-insoluble compounds remaining in the particles.
In summary, each of the foregoing methods is defective in one or more respects. Although the first method (successive seeded emulsion polymerization) provides monodisperse latexes in high concentrations, the particle size is limited to about 2 microns and even at these sizes, either a new crop of small particles is nucleated or excessive coagulum is formed. The second method (emulsifier-free emulsion polymerization) produces monodisperse latexes only in low concentrations (10% or less) and particle sizes smaller than 1 micron. Even when the technique is combined with seeding, the maximum size particles that can be produced is only 4 microns. The third technique (microsuspension) produces a broad distribution of particle sizes and requires tedious and time-consuming fractionation to give narrow particle size distribution. Even then, the particle size distributions of the fractionated latexes are broader than those of the monodisperse latexes prepared by seeded emulsion polymerization. The fourth technique (high-swelling seeded emulsion polymerization) gives latexes of poor uniformity, and their use as size standards is limited by their tendency to explode upon heating or expo-sure to an electron beam. Moreover, this method requires separation of the crop of new small particles as well as the off-size larger particles to give a latex of narrow particle size distribution.
U.S. Pat. No. 4,247,434 discloses further improvements upon the successive seeded emulsion polymerization technique by practice of the process in a microgravity environment such as has been made available by NASA space shuttle missions. The microgravity environment avoids gravity-related problems such as settling and creaming due to density differences during polymerization, and thereby promotes the production of monodisperse particles larger than 2 microns. Despite the promise of the microgravity process, problems still persist in efforts to prepare monodisperse latexes having the larger particle sizes. These include a trade-off between larger particle size and concentration thereof (the concentration of larger particles decreases as particle size increases, due to formation of a new crop of small particles and/or excessive coagulum), over-size particle formation from particle coalescence, and continuing tendencies toward creaming and settling.